This invention relates to ceramic composites that are tough, yet display exceptional resistance to thermal shock. More particularly, this invention relates to an alumina-boron nitride dielectric ceramic that displays superior mechanical as well as thermal shock and ablation resistance. Such a composite may be utilized as a structural material in the fabrication of radomes and antenna windows, missile and rocket components, and other related structures which encounter high temperatures during use and are not exposed to long periods of oxidation at extreme temperatures.
The fabrication of ceramic materials characterized by exceptional resistance to thermal shock, ablation, weather erosion, and the like has become a technology of significant importance. A number of materials have been suggested and tried in an attempt to find such a suitable material. For example, among the current conventional radome materials, alumina and "Pyroceram 9606", crystalline glass-like ceramic sold by Corning Glass, Inc., have been clearly demonstrated not to meet all these advanced requirements. Fused SiO.sub.2, while having adequate resistance to thermal stress fracture, has inadequate ablation resistance due to its limited refractory character, as well as significant weather erosion deficiencies. Si.sub.3 N.sub.4 has extreme thermal environment limitations, while boron nitride is extremely expensive as well as mechanically weak.
Important elements in improving a composite's thermal-stress resistance in extreme high-temperature environments are to reduce the thermal conductivity of the material and increase the strain tolerance. Introduction of a second phase material into the crystalline microstructure can significantly reduce the thermal conductivity of a material. The size and thermal conductivity of these second phase particles are extremely important composite parameters. In particular, the particle size and thermal conductivity of the second phase material determines the character of the immediate microstructure of the composite, i.e., the magnitude and number of microcracks which result upon thermal expansion of both composite materials when exposed to significant temperature fluctuations. Generation of microcracks is an important strain accommodating mechanism, and hence increases strain tolerance.
The prior art has attempted to solve the problem of increased resistance to thermal shock by placing the emphasis on inhibiting or arresting crack propagation. A recent theoretical evaluation, i.e., Rossi (Thermal Shock-Resistant Materials, in "Ceramics in Severe Environments", ed. N.Y. Planum press, 1971, pp. 132-134), discusses the theory of thermal crack propagation and prevention and evaluates a hot-pressed BeO matrix containing 10 volume percent of pyrolytic BN, creating a thermal-resistant, hypereutectic structure. However, the BN phase particles are much larger than the BN particles used in this composite, being platlets of 75-350 microns diameter and no unusual importance is attached to their exact size. In U.S. Pat. No. 4,007,049, Rossi et al., the alumina-boron nitride system is evaluated, but only for composites containing from about 5 to 30 volume percent of these large BN particles. Furthermore, the patent teaches that the incorporation of powdered boron nitride does not provide the necessary increase in thermal shock resistance which the dispersed phase having a flake morphology achieves.